JPH0751813A - Continuous casting method and its apparatus - Google Patents

Abstract

PURPOSE:To improve the deterioration in quality of the top part of a cast slab caused by catching of inclusion into the cast slab and to reduce the length of solidification shrinkage hole (shrinkage pipe) caused by the solidification and the shrinkage after completing the casting. CONSTITUTION:A device 8 for heating near the molten steel surface in a mold 4 is arranged in a continuous casting apparatus and the molten steel near the molten steel surface is heated during a period from the stage at the end of casting when the molten steel temp. supplied into the mold from a tundish 2 lowers to the stage when a cast slab perfectly solidifies. The molten steel temp. in the mold is risen during casting to accelerate the floating-up and separation of the inclusion suspended in the molten steel. After completing the casting, the molten steel surface is kept to the molten state and the shrinkage pipe depth developed at the cast slab top part is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention aims to improve the quality of a slab top portion in continuous casting, suppress solidification shrinkage holes (shrinkage pipes) in the slab top portion, and improve the quality of the slab. It is about improving the stay.

[0002]

2. Description of the Related Art At present, in steel production, the production ratio by continuous casting has already reached a level exceeding 90%.

However, there are some steel types that have not been continuously cast because of the fact that continuous casting is difficult due to their components, the order lot is small, the casting size is large, and the casting speed cannot be increased.

Since these cannot be efficiently incorporated into an existing high productivity continuous casting machine, they are produced by the ordinary ingot casting method, which is inferior to the continuous casting method in terms of cost and productivity.

On the other hand, even steel types already produced by continuous casting machines tend to be stricter in quality, have a wider variety of products, and have a smaller lot due to diversification of user needs. There are also steel types that must be manufactured.

[0006] These steel types can be incorporated in continuous casting production by performing continuous casting of different steel types. However, workability and productivity are impaired due to insertion of a separated iron plate for suppressing component mixing, It is not possible to sufficiently suppress the mixing of the components of the seam of the steel type, and it is necessary to scrap the component mixing part.

Alternatively, in the case of incorporating a small lot material below the manufacturing lot of a refining furnace for supplying molten steel to continuous casting, there is a problem that the surplus material accompanied by a storage fee increases.

Therefore, for the purpose of eliminating the disadvantage of producing a small lot material by a high production continuous casting machine, Japanese Patent Publication No. 4-5742.
There has been proposed a semi-continuous type continuous casting machine exclusively for small lot materials as seen in "Continuous casting machine for large-section cast slab" in Japanese Patent Laid-Open No. 25-25.

A problem in the operation of the continuous casting machine dedicated to small lot materials is that when the continuous casting is carried out in a large lot with the cut-off ratio of the top and bottom portions inevitably occurring in each casting lot because the casting lot is small. However, it is necessary to take measures to reduce the amount of cut-off of these parts.

The temperature of the molten steel used for continuous casting is adjusted by refining and secondary refining, and the molten steel pouring temperature is controlled uniformly in the tundish.

As a method therefor, there are induction heating, plasma heating, etc. for the purpose of heat compensation at the start and end of pouring. Attempts have been made to adjust the temperature to a casting target temperature and make it uniform by these methods.

However, like the molten steel temperature in the tundish shown in FIG. 1, the molten steel temperature tends to change drastically at the start and end of pouring as compared with the temperature level in the middle.

In the figure, the solid line shows the behavior of the molten steel temperature in the tundish without tundish heating, and the alternate long and short dash line shows the behavior of the molten steel temperature with tundish heating.

The tundish heating is applied in the latter half of the pouring of the molten steel in the pot where the temperature of the molten steel in the pot to be poured into the tundish decreases, but the induction heating and plasma heating devices currently used lower the molten steel level in the tundish. In that case, the heating is difficult and there is a drawback that the decrease in the temperature of the molten steel of the tundish cannot be improved (the non-heating range is shown by an arrow in the figure).

The low temperature region of the molten steel at the start of pouring the molten steel in the tundish is mainly due to the heat absorption by the refractory of the tundish, and the temperature lowering region just before the end of the pouring is the decrease of the temperature of the molten steel injected into the tundish (in the pot). The heat dissipation from the molten steel temperature in the tundish), the heat dissipation from the tundish refractory, and the deterioration of the heating characteristics due to the lowering of the molten metal surface inside the tundish, which can be prevented only by the conventional technology of equalizing the molten steel temperature injected from the inside of the tundish. Can not.

Therefore, the low temperature molten steel is injected into the mold at the start of the molten steel injection from the tundish and immediately before the completion of the injection.

Further, non-metallic inclusions dispersed in molten steel due to deoxidation of steel, melting of refractory materials such as pots and tundish are pots,
It shows a behavior of floating while molten steel stays in the tundish, but it cannot be completely floated during casting and casting, and molten steel with a relatively high concentration of inclusions is injected into the mold at the end of casting. Become.

The floating of inclusions in the molten steel is influenced by the molten steel temperature, and the higher the molten steel temperature, the easier the floating.

FIG. 2 shows the slab quality caused by inclusions at each slab site. It can be seen that the slab top portion (T) having a low molten steel temperature is inferior in quality to the slab bottom portion (B) as well as the slab middle portion (M) and the slab joint portions (R, S).

The cause of the quality deterioration of the slab top (T) is considered to be that the molten steel containing a large amount of inclusions suspended at a low temperature is injected immediately before the completion of casting, and the casting temperature is properly adjusted while cleaning the molten steel. It is considered to be effective to maintain it.

FIG. 3 shows the molten steel temperature in the tundish and the defect generation rate due to inclusions. Target temperature 155 for this steel type
It can be seen that the defect occurrence rate increases when the temperature deviates from the range of 0 ° C. ± 10 ° C. to the lower temperature side.

4 to 5 show solidification shrinkage holes (shrinkage pipes). After the casting is completed, the molten steel is not supplied from the tundish (and therefore the heat is not supplied).
With the solidification from the side surface of the slab due to the subsequent cooling, the heat dissipation from the molten metal surface also causes the solidification to proceed from the molten metal surface side.

Solidification from the head of the slab (on the side of the molten metal) is caused by solidification and shrinkage of the unsolidified portion inside the slab on the side of the molten slab.
This hinders the supply of molten steel from the steel, resulting in the formation of deep solidification shrinkage holes (shrinkage pipes) in the head of the slab.

Therefore, in the conventional continuous casting method, 300 to 5 is used.
In the case of a bloom slab with a shape close to a square cross section of about 00 mm square, the solidification shrinkage hole length is 1.2 to 1.6 m from the slab head (melt surface position), like a flat slab with a thickness of 200 to 250 mm. In the case of a rectangular cross-sectional shape, the depth is about 0.4 to 0.6 m, and the cut-off amount of the top portion is at least a length commensurate with the generated shrinkage pipe (Figs. 4 to 5:
L).

Also, since molten steel containing a large amount of inclusions suspended at a low temperature is injected immediately before the completion of casting, there are some varieties with a cut-off amount of 4 m or more due to quality restrictions due to inclusions in the slab (Fig. 6). .

As a method for heating the molten steel in the mold, a method of immersing a high frequency heating coil in the mold in JP 62-224466 A, JP 61-245949 A, JP 62-224454 A is used. A method of mounting a heating element in the vicinity of the mold cooling plate of the publication is proposed, but the former is inferior in maintainability of the heating element because the heating element is directly immersed in molten steel, the life is short, and the latter is a special mold. There are drawbacks such as the need for structure.

Further, as a method of reducing the length of the shrinkage pipe, the reduction device installed directly below the mold disclosed in Japanese Patent Laid-Open No. 2-268954 reduces the width of the cast slab in the width direction, thereby reducing the shrinkage pipe. There is proposed a method of reducing the number of the materials, or a method of using a heat insulating frame and a heat generating heat insulating material in the slab top portion of Japanese Patent Laid-Open No. 63-157744.

In the former case, a mold, a mold vibrating device, and the like are intricately assembled, and it is necessary to install a pressure reducing device at a position that is easily damaged by breakout or the like. On the other hand, the latter has the drawback that automation is difficult.

The present invention proposes a method and apparatus capable of compensating for the above-mentioned drawbacks, improving the quality of the slab top portion with the same equipment, and reducing the shrinkage pipe.

[0030]

SUMMARY OF THE INVENTION According to the present invention, in the final stage of continuous casting, molten steel containing a large amount of inclusions is injected into the mold at a low molten steel temperature, and the inclusions are collected in the slab. (EN) Provided are a casting method and an apparatus suitable for improving the deterioration of quality of a top cast piece caused by the above, and reducing the solidification shrinkage hole (shrinkage pipe) length generated by solidification and shrinkage at the time of completion of casting.

[0031]

Means for Solving the Problems The gist of the present invention is that the molten steel injected or the surface of the molten steel in the mold can be heated or kept at the end of casting during continuous casting, and the decrease of the molten metal level can be followed. By arranging a heating device that can heat the surface of molten steel,

1) At the end of continuous casting, the surface of the molten steel in the mold is heated to raise the temperature of the molten steel at the end of pouring, or to keep it warm to promote the floating of inclusions in the molten steel and improve the quality of the top of the slab. To do.

2) At the end of casting, the molten steel surface in the mold is heated or heat-retained, and finally solidification is completed on the molten steel surface in the mold. Is suppressed from occurring in the deep portion, and the yield of the cast piece is improved.

The slab top portion refers to the slab at the portion corresponding to the casting completion portion of the slab manufactured in continuous casting, and the end of casting refers to the time after the molten steel temperature starts to drop at the end of casting. .

Plasma heating devices and gas heating devices can be considered as the molten metal surface heating device applicable to 1) and 2).

An induction heating device for directly heating the injection flow flowing down from the tundish can be considered as a device for heating only the injection molten steel at the final stage of injection.

[0037]

EXAMPLE FIG. 7 is a view of the continuous casting machine during casting as seen from the short side of the slab, and the details of the present invention will be explained based on this figure.

Molten steel is poured into the tundish 2 from a refining vessel (not shown), and the immersion nozzle 3 is fed from the tundish.
Through the mold.

The injected molten steel is cooled on the surface of the mold, forms a solidified shell, and is drawn out while the unsolidified molten steel is held by the solidified shell.

After the mold, the solidified shell is held by the rolls 6 and further cooled by the secondary cooling device 7 installed between the rolls to reach a completely solidified state.

Now, the casting method of the present invention with the above equipment structure will be described.

1) Operating Method for Quality Improvement of Top Piece Site The operation method for improving the quality of the top piece is to control and maintain the molten steel temperature in the mold at the target temperature when the molten steel temperature in the tundish decreases just before the completion of casting. This is the technical point above.

Therefore, the molten steel is heated by the molten steel heating device 8 so as to increase the temperature so as to compensate for the decrease in the temperature of the molten steel injected from the tundish into the mold and the heat radiation from the molten steel surface in the mold.

2) Operating Method for Reducing Shrinkage Pipe during Solidification In order to reduce the shrinkage pipe, the heat radiation from the molten steel surface in the mold is compensated to prevent the solidification from the molten steel surface. The molten steel temperature is maintained by the heating device 8, and if a decrease in temperature is recognized, heating is performed.

Further, since completion of solidification on the surface of molten steel in the mold is a technical point that enables reduction of the shrinkage pipe, cooling immediately below the mold is continued during heating of the molten metal surface, and the solidification tip is kept at the molten metal surface. Perform an operation to raise to the side.

The heating device is a device capable of following the deterioration of the molten metal level, and in consideration of seizure of the slab with the mold, or bulging immediately below the mold, drawing is performed at an extremely low speed during heating. Can also be adopted.

Table 1 shows the operating conditions, and FIG. 8 shows the operating method.

[0048]

[Table 1]

In adopting a heating method from the molten steel surface such as plasma heating this time, the existing electromagnetic stirrer 5 is also used in consideration of equalizing the molten steel temperature in the mold, and the molten steel is heated while the molten steel is being heated. Stirring was performed.

9 and 10 show the improvement effect. It was possible to reduce the UST defect rate due to inclusions by a third and the depth of the top piece by 100 mm as compared with the conventional method.

[0051]

EFFECTS OF THE INVENTION According to the present invention, the vicinity of the molten metal surface is heated to promote the floating separation of inclusions in the top slab, and at the same time, the molten metal surface is maintained to maintain the quality of the slab top. , Improve the shrinkage pipe depth.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a molten steel temperature behavior in a tundish during casting.

FIG. 2 is an explanatory diagram showing the quality of each cast piece part.

FIG. 3 is an explanatory view showing the influence of the molten steel temperature on the quality of the slab.

FIG. 4 is an explanatory diagram showing a tendency of occurrence of a shrinkage pipe (at the end of injection).

FIG. 5 is an explanatory diagram showing a tendency of a shrinkage pipe to occur (at the time of complete solidification).

FIG. 6 is an explanatory diagram showing the tendency of inclusion-type defects to occur in the slab top portion.

FIG. 7 is an explanatory view showing an embodiment of the present invention.

FIG. 8 is an explanatory diagram showing an operating method.

FIG. 9 is an explanatory diagram showing quality improvement results.

FIG. 10 is an explanatory view showing a result of improvement of the shrinkage pipe depth.

[Explanation of symbols]

 1 Molten Steel Heating Device (Tandish) 2 Tundish 3 Immersion Nozzle 4 Mold 5 Electromagnetic Stirring Coil 6 Roll 7 Secondary Cooling Device 8 Heating Device (In Mold) 10 Solidification Nucleus 12 Unsolidified Molten Steel

Claims (3)

[Claims] 1. A continuous casting method, characterized in that casting is performed while heating or heat-retaining the surface of the molten steel in the mold at the end of casting during continuous casting. 2. A continuous casting method, characterized in that the surface of the molten steel in the mold is heated or kept at the end of casting to complete solidification on the surface of the molten steel. 3. A continuous casting characterized by comprising a heating device capable of heating the molten steel surface in the mold at the end of casting during continuous casting and capable of heating the molten steel surface in accordance with the decrease of the molten metal surface. apparatus.